Method of rounding a top corner of trench

ABSTRACT

A method for pull back S i N to increase rounding effect in a shallow trench isolation process, includes the acts of preparing a substrate of Si and forming a SiO 2  layer on the substrate, forming a Si 3 N 4  layer on the SiO 2  layer, defining Si 3 N 4  trenches by plasma etching; etching the remaining Si 3 N 4  with SF 6 /HBr gas, etching SiO 2  layer to form a platform and enhance the rounding of the platform, etching the substrate to have a third shallow trench and a reinforced platform, filling the third shallow trench with oxide, leveling the oxide, which uses a CMP to level the filled oxide, and removing the Si 3 N 4  layer, wherein after the removal of the Si 3 N 4  layer, multiple cleaning processes are required.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for increasing rounding effect in a trench top comer, and more particularly to a method for pull back S_(i)N to increase rounding effect in a shallow trench isolation process (STI) and to avoid Wrap Round in the trench top comer.

BACKGROUND OF THE INVENTION

[0002] Trench isolation is a widely used method in the modem VLSI process to isolate elements. The primary principle of the technique is to use the anisotropic dry etch to define a trench and then a filling is added to the trench. As the trend of the modem semiconductor, the dimension of the elements used in the semiconductor is smaller and smaller, which causes the dimension of the isolation layer to decrease. Accordingly, semiconductors having the structure of STI become the mainstream.

[0003] With reference to FIGS. 1A to 1H, the conventional STI technique includes the following acts. It is necessary to form a S_(i)O₂ layer 102 on a substrate 101 of S_(i). Then it is required to form a S_(i3)N₄ layer 103 on the S_(i)O₂ layer 102 to form a shallow trench area 104. Then, using high density plasma chemical vapor deposition to precipitate an oxide 105 in the shallow trench area 104 is the next act. After the precipitation act, it is necessary to level the oxide 105. Rounding the shallow trench corner 104 a would be the next act. Then, it is necessary to remove the S_(i3)N₄ layer 103. Finally, cleaning, including wet cleaning, cell cleaning and tunnel cleaning is required.

[0004] In the conventional method, when using an etcher to level the oxide 105 and after the post cleaning process, an oxide recessed portions 106 will be formed on the edge of the oxide 105 in the shallow trench area 104 and the wafer to cause abnormal conductivity, such as double hump in the I_(d)-V_(G) curve.

[0005] Therefore, it is necessary to provide an improved method to form a shallow trench isolation structure without worrying the formation of the wrap round.

[0006] To overcome the aforementioned problem, some introduces a method, as shown in the prior art and in FIGS. 2A to 2I, to protect the edges of the wafer, which includes the following acts.

[0007] It is necessary to form a S_(i)O₂ layer 202 and a S_(i3)O₄ layer 203 on a substrate 201 of Si. Forming a photo resisting layer 204 that defines an opening 205 above the S_(i3)O₄ layer 203 would be the second act. Then, anisotropic etching the SiO₂ layer 202 and the S_(i3)N₄ layer 203 would be the third act. The fourth act is to form an encasing wall 206 a around the photo resisting layer 204, the SiO₂ layer 202, the S_(i3)N₄ layer 203 and the opening 205. The fifth act is using dry etching to etch the encasing wall 206 b that encloses the opening 205 and the substrate 201 to form a shallow trench area 207. Then, it is necessary to remove the photo resisting layer 204 and the encasing wall 206 b to expose the unetched area on the substrate 201 and a sharp edge 208 a. The eighth act would be forming an oxide of Si 209 on the unetched area of the substrate 201 and the sharp edge 208 a to change the sharp edge 208 a to round edge 208 b. Then, precipitating an insulation layer 210 on the oxide of Si 209 and filling the shallow trench area 207 would be the ninth act. Finally, it is necessary to remove the SiO₂ layer 202 and the S_(i3)N₄ layer 203 on the insulation layer.

[0008] This method uses the encasing wall 206 b of a polymer to fill in the opening 205. When the polymer is removed and the insulation layer 210 is filled in the space left by the removal of the polymer, the insulation layer 210 is able to protect the comer 211.

[0009] This method does provide the necessary requirements, however, it contains too many acts and acts such as forming the encasing wall and removal of the encasing wall will inevitably increase the cost.

[0010] According to the foregoing technique, the methods such as wet etch or oxidation to pull back the SiN complicates the process and increases the cost. Furthermore, after the S_(i3)N₄ layer is removed, the post cleaning process easily forms wrap round on the trench top corner and thus causes high electric field and pre-breakdown.

[0011] To overcome the shortcomings, the present invention intends to provide an improved method for pull back S_(i)N to increase rounding effect in a shallow trench isolation process and to avoid Wrap Round in the trench top corner.

SUMMARY OF THE INVENTION

[0012] The primary objective of the invention is to provide a method for pull back S_(i)N to increase rounding effect in a shallow trench isolation process and to avoid Wrap Round in the trench top corner.

[0013] In order to accomplish the foregoing objective, the method adds an isotropic etching process to pull back the Si₃N₄ and to increase the trench top corner rounding. That is, after the SiN layer is etched to a predetermined depth, a gas of SF₆/HB_(r) is applied to fully etch the remaining SiN. With the gradient variation of the depth by the SF₆/HB_(r), and the etch rate selectivity of SiO₂ to Si being less than 1, a top rounding etch act is adopted to continue etching the oxide layer and the substrate to have a reinforced top comer rounding.

[0014] Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIGS. 1A to 1H are schematic views of a conventional method used in the STI.

[0016]FIGS. 2A to 2I are schematic views of another conventional method in the STI, wherein an oxide recessed portions will be formed on the edge of the oxide in the shallow trench area and the wafer to cause abnormal conductivity.

[0017]FIGS. 3A to 3I are schematic views of the method of the invention, wherein an isotropic etching process is used to pull back the Si₃N₄ and to increase the trench top comer rounding.

DETAILED DESCRIPTION OF THE INVENTION

[0018] With reference to FIGS. 3A to 3I, a method of the present invention using an isotropic etching process to pull back the Si₃N₄ and to increase the trench top comer rounding is shown.

[0019] The method comprises the following acts. It is necessary to prepare a substrate of Si 301 and form a SiO₂ layer 302 on the substrate 301. Then, it is necessary to form a Si₃N₄ layer 303 on the SiO₂ layer 302. The third act is to define shallow trenches 304 a by etching, which applies a photo resisting layer 305 on the Si₃N₄ layer 303 and then partial etches the Si₃N₄ layer 303 with plasma etcher to form shallow trenches 304 a. The fourth act is to etch the remaining Si₃N₄ layer 303 with SF₆/HBr gas; which uses a plasma etch at same process chamber (in-situ) or the plasma etch may be processed in another etcher (ex-situ), wherein because SF₆/HBr gas has high etching selectivity of the Si₃N₄ to oxide, such that during the SF₆/HBr gas soft landing on the SiO₂ 302 layer, the SF₆/HBr gas pushes the Si₃N₄ layer 303 backward and etches SiO₂ layer 302 to form depth gradient so as to form a trench 304 b. The fifth act is to use CH₄/HBr, CF₄/CH₂F₂, CF₄/CHF₃, CH₂F₂ or CHF₃ gases to continue etching SiO₂ layer 302 and part of substrate such as 300˜500 Å so as to enhance the rounding of the platform 305. Then, it is required to etch the substrate 301 to have a shallow trench 304 c at a depth of about 5000 Å and a reinforced platform 305. Filling the shallow trench 304 c with oxide 306 would be the next act. Then, after the filling act, it is necessary to level the oxide 306, which uses a chemical mechanical polish to level the oxide 306. Finally, it is required to remove the Si₃N₄ layer 303. After the removal of the Si₃N₄ layer 303, multiple cleaning processes are required.

[0020] In the dry etching act of the present invention, it may be applied at poly etcher with in-situ Si₃N₄ open and has the least time and least cost to complete the process when compared with the foregoing conventional method. The method is able to make an extended silicon platform 305 after the shallow trench isolation process is finished to avoid wrap round. Especially, with or without the rounding act, the method of the invention can still protect the STI corner to avoid abnormal conductivity. Therefore, in the post trench isolation is finished, wrap round at the STI comer is avoided.

[0021] Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A method of rounding a topcorner of a trench in a shallow trench isolation process, comprising the steps of: a. preparing a substrate of Si and forming a SiO₂ layer on the substrate; b. forming a Si₃N₄ layer on the SiO₂ layer; c. by partionally etching the Si₃N₄ layer with plasma etcher to form a first-stage trench; d. etching the remaining Si₃N₄ layer under the first-stage trench with SF₆/HBr gas and etching SiO₂ layer to form depth gradient so as to form a second stage trench; e. etching SiO₂ layer, wherein a platform is formed; and rounding the platform, using a top rounding etch act to enhance the rounding of the platform; f. etching the substrate under the second-stage trench to have a completed shallow trench having a rounded top comer.
 2. The method as claimed in claim 1, wherein a gas used to etch the SiO₂ layer is selected from the group consisting of CH₄/HBr, CF₄/CH₂F₂, CF₄/CHF₃, CH₂F₂ or CHF₃. 